1. Field of the Invention
This invention relates generally to monolithic series-connected solar cells and, more particularly, to interdigitated thermophotovoltaic power converters. Specifically, the present invention relates to an improved front-back interdigitated contact arrangement for use in such monolithic series-connected power converter devices.
2. Description of the Prior Art
In general, a semiconductor solar cell includes a plurality of p and n conductivity-type regions in a semiconductor body. These regions generate a voltage potential and/or a current when electron-hole pairs are created in the semiconductor body in response to impinging radiation on the top or emitter layer. When this occurs, the holes and electrons migrate, respectively, to p-doped and n-doped regions. Because of the typical small voltage generated for each cell, for example 0.5 to 0.8 volts open circuit for a silicon concentrator cell, such cells are serially connected to achieve higher operating voltages. In monolithic structures where the cells share a common substrate, electrical isolation as well as serial connection of the individual cells must be provided.
Thermophotovoltaic (TPV) systems convert radiant energy and infrared light energy from a heat source into electricity in much the same manner as a traditional solar cell converts visible light from the sun into electricity. Since the energy spectrum emitted from a heat source is dependent on the temperature of the source, and since these temperatures are generally much lower than the temperature of the sun, thermophotovoltaic converters must use semiconductors with band gaps that are much smaller than those traditionally used for solar cells. The low band-gaps of the TPV converter insures that the voltage generated by individual discrete devices is typically less than approximately 0.5 volts. Additionally, the close proximity of the converter to the radiant energy source with its high flux density, results in the generation of extremely high current densities in such converters. These high current densities combine with low voltage outputs to cause a large fraction of the generated power to be dissipated by resistance losses in the front surface metallization of the converter. Since the generated current scales with the area of the device, these parasitic power losses result in a practical upper limit for the size of an individual discrete device of around 1 cm.sup.2. Because many proposed thermophotovoltaic applications envision a relatively large area converter array, the practical considerations associated with considering such an array with 1 cm.sup.2 individual devices provide incentive for developing monolithic integrated circuits for these devices that will allow an entire wafer to be processed into a single component. Additionally, the proposed array is fabricated on a semi-insulating substrate facilitating incorporation of an effective back surface reflector, which is a necessary element of an efficient thermophotovoltaic system.
All of the current approaches to the fabrication of monolithically interconnected thermophotovoltaic converters share a common design element. They assume the active device layers are grown epitaxially on a semi-insulating substrate, commonly in InP. The purpose of the semi-insulating substrate is two-fold. First it allows for the electrical isolation of the individual discrete devices, that is isolation in individual cells on the device. Secondly, the lack of free-carriers in the substrate should facilitate the incorporation of an effective back-surface reflector. Since the front and back contact metallization is required to be located on the front or top surface of the cell due to the back surface reflector layer, the metallization utilized for such front-back contacts tends to reduce the amount of surface area available for exposure to impinging radiation. Such front-back contact structures on the top surface of the device are also applicable to general solar photovoltaic cells to permit easy mounting of such cells into a device. Heretofore, a number of monolithic solar cell configurations having front-back contact arrangements have been designed. Some examples of such devices are illustrated in U.S. Pat. Nos. 4,865,999 and No. 4,933,021. Typical solar cell configuration includes contacts disposed on the back surface of the cell without a back surface reflector layer such as illustrated in U.S. Pat. No. 4,838,952. To date, front-back contact arrangements for photovoltaic converters have provided inadequate performance for high current operation especially when utilizing devices having lattice-mismatched compositions due to series resistance losses from the contact structure. Consequently, there is still a need for a photovoltaic power conversation device which includes an interdigitated front-back contact arrangement that provides high efficiencies and low series resistance in the cell.